This invention relates generally to digital signal processing and, more particularly, the invention relates to the processing of multiple direct-sequence spread-spectrum signals such as in Global Positioning System (GPS) receivers, for example.
In a GPS navigation system, there are a plurality of GPS Satellite Vehicles (SVs) orbiting the earth. Each broadcasts a direct sequence spread spectrum signal at frequency L1. By receiving a plurality of these signals from in-view GPS satellite vehicles, very accurate location and navigation information can be determined almost anywhere on or above the earth that is visible to the GPS satellites.
GPS position solutions generally require processing at least four signals. Degraded solutions are possible with fewer signals, but the integrity, availability, accuracy, and speed of GPS positioning systems improve strongly as more signals and processing elements are added to the system. Market competition is driving high-performance GPS receivers to incorporate steadily increasing numbers and kinds of signals and signal processing elements. For example, one L1-only GPS receiver has the equivalent of 72 complex correlator channels.
The number and kind of GPS-like signals which such receivers could process is growing. GPS Svs have always transmitted similar signals at frequency L2, albeit encrypted for military use. Block IIF GPS Svs will likely provide an additional frequency for civilian use. The Russian GLONASS navigation system employs multiple carriers (1600 to 1615 Mhz). Europe is planning to establish its own analog to GPS: GNSS. The FAA is planning to establish a system of Wide Area Augmentation System (WAAS) signals via satellite, using one or more carriers at or near L1. Inmarsat and probably others are planning similar systems.
There is also a growing appreciation of the need to augment GPS from the ground using Pseudolites (PLs). Various designs have been proposed, including in-band (at or near L1) and out-of-band (away from L1) PL carriers. The FAA LAAS program, for example, contemplates adding in-band Pls at airports to improve the integrity of the GPS system. Commercial entities may provide networks of out-of-band Pls, both for integrity, and also to counter blockages near large ships, bridges, or warehouses.
Thus, it is technically preferable for GPS receivers to process all available GPS-like signals, the market is driving high-performance receivers in this direction, and there is a potential explosion in the number of GPS-like signals available for use in positioning systems. Therefore, novel techniques for efficiently receiving large number of such signals will be advantageous.
Current and previously proposed technology allow for the tracking of signals at both the L1 and L2 frequencies defined by the NAVSTAR GPS system. Extensions have been made to cover the Russian GLONASS navigation system by 3S Navigation, e.g. U.S. Pat. Nos. 5,311,194, 5,225,842 and 4,754,280.
A. J. Van Dierendonck, "Section 8: GPS Receivers", Global Positioning System: Theory and Practice, Volume 1; Progress in Astronautics and Aeronautics, Vol. 163, AIAA, 1996, provides a concise, reasonably complete discussion of modern GPS receivers. FIG. 1 shows a typical digital GPS receiver, which contains the following components: antenna 1, preamplifier 2, reference oscillator 3, frequency synthesizer 4, downconverter 5, intermediate frequency (IF) section 6, signal processing 7; and applications processing 8. All receiver components except for signal processing are typically common for all received signals.
FIG. 2 depicts a typical conventional GPS receiver, the MAGR. Here, the signal processing functions are further decomposed into sampling 11, carrier NCO 12, doppler removal 13, code NCO 14, reference code generate 15, correlation 16, sum-of product accumulation 17, and demodulation/tracking functions 18. Conventional receivers typically use a common sampler to generate a common sample stream for all signals. On the output side, the demodulation/tracking functions are typically implemented to microprocessor firmware, and thus employ common logic.
Existing GPS Receivers typically use one of three main approaches for the intermediate digital processing of multiple signals. The first is called sequencing, the second is called dedicated channel tracking, and the third is called multiplexing. Sequencing is defined by sharing one physical receiver channel to track multiple signals, one at a time, whereas the dedicated channel method uses a single channel for each signal that is tracked. Multiplexing falls in between.
Only the dedicated-channel approach is appropriate for high-performance GPS receivers. In this approach, FIG. 2 is best understood as showing a single channel of such a receiver.
FIG. 3 depicts the Stanford Telecom 9550 Triple Correlator. A conventional n-channel GPS receiver can be implemented using n of these units. Each STEL-9550 correlates its sampled inputs with the externally-supplied reference code, so n code generator functions would also be needed to generate the reference codes. The STEL-9550 processing approach differs from the conventional design in that the doppler removal is done after correlation. In normal use, decimation by a factor of 12 is provided prior to doppler removal. This enables the STEL-9550 to perform doppler removal and accumulation functions by multiplexing a single scalar multiply/accumulate unit to continuously update three complex sum-of-product accumulators. This key concept (correlate, decimate, then multiplex the rest) foreshadows the invention. The STEL-9550 however, does not take this beyond a single signal, and so it does not reduce overall complexity much, if at all, relative to the conventional approach.